The invention relates to a method for cleaning a filter element located in a filtering chamber, in which, after interruption of the filtering flow from the filtering chamber inlet side through the element to the outlet side of the chamber, the chamber inlet side is connected with a lower pressure in order to generate a pressure difference instantaneously between the inlet and outlet side of the element and in order to generate a flow impulse for detachment of solid matter accumulated on the element surface. Additionally, the invention relates to a filter to which said method is applicable.
Continuously operating automatic filters, whose operation is based on alternating filtering and backwashing steps, are used as fuel and lubricant filters in diesel engines, for instance. Filtering removes solid impurities from the flow being filtered, and the function of repeated flushing steps is to keep the filter element in operating condition.
A typical automatic filter comprises two or more filter elements coupled in parallel, each of which is located in an individual filtering chamber. The filter elements are used alternatingly, so that, while the flow to be filtered is being directed to the one element, the other element is cleaned by backwashing, until rotation of their common inlet valve for flow to be filtered reverses these operation steps.
Filtered liquid conducted from the filter outlet side can be used for backwashing of the filter elements, the liquid being under the operating pressure of the filter, i.e. under the pressure of the filtered discharge flow. Connecting the inlet side of the filtering chamber to lower pressure activates flushing, and then flushing takes place by means of the pressure difference between the operating pressure of the filter and said lower pressure. Usually the pressure difference is achieved by connecting the inlet side of the filtering chamber to atmospheric air pressure. A sudden pressure drop on the inlet side of the chamber generates a pressure difference between the inlet side and the outlet side of the filtering chamber, this pressure difference, in turn, generating a pulse in the flushing flow so that any solid impurities accumulated on the inlet side of the element surface are detached and washed out of the filter by the flow. Filters are used in which flushing takes place exclusively by means of a filtered liquid recovered on the filter outlet side, and also filters whose filtering chamber is emptied of liquid as the flushing step starts, after which flushing is continued by means of compressed air. FI lay-out print 107127, for instance, describes an automatic filter of the latter type, in which the filtering chamber has been connected with an air tank, the flushing air compressed in this tank enhancing the liquid discharge from the chamber at the beginning of each flushing step.
So far, automatic filters have been used in low and medium-pressure systems, in which the prevailing pressure is generally 20 bars at the most. The filter elements have been devised so as to resist a pressure difference of this order after the filtering chamber has been connected with atmospheric air at the beginning of the flushing period. At the same time, the pressure difference is sufficient for effective detachment of solid matter from the surface of the filter element. However, it would be desirable to be able to use automatic filters operating on the backwashing principle also in high-pressure hydraulic systems, whose typical operating pressure is of the order of 250 bars, for instance. Connecting a filtering chamber under such pressure to atmospheric pressure in order to achieve flushing, would, however, cause damage to the filter element. In order to resist a momentary pressure difference of 250 bars, the filter elements should be given a very durable construction, which would entail high costs. The pressure prevailing in the filtering chamber could be decreased gradually under control by means of pressure-measuring sensors and adjustable valves mounted in the reject duct for discharge from the chamber, yet this would entail complex and expensive constructions, and would still not easily allow for adequate repeatability accuracy of the pressure-drop steps. An occasional excess of the set value for the pressure drop could immediately result in damage to the filter element.